BUG: Fix error raised when editing a public study
BUG: Fix error raised when editing a public study

If a public study was to be edited, a KeyError would be raised as the
environmental_packages attribute is non-editable once the study is
viewable by everyone.

Fixes #816

RNAer/qiita,squirrelo/qiita,adamrp/qiita,RNAer/qiita,josenavas/QiiTa,ElDeveloper/qiita,josenavas/QiiTa,biocore/qiita,adamrp/qiita,josenavas/QiiTa,squirrelo/qiita,biocore/qiita,ElDeveloper/qiita,josenavas/QiiTa,ElDeveloper/qiita,antgonza/qiita,squirrelo/qiita,ElDeveloper/qiita,antgonza/qiita,biocore/qiita,antgonza/qiita,wasade/qiita,antgonza/qiita,adamrp/qiita,RNAer/qiita,wasade/qiita,adamrp/qiita,RNAer/qiita,squirrelo/qiita,biocore/qiita,wasade/qiita